Nicolet1 and Urey2 have made estimates of such physical parameters of the atmospheric escape layer as the height of this critical level, its temperature, and the corresponding scale height, by extrapolation of results of the Rocket Panel3. Since the data of the Rocket Panel extend to a height of only 220 km., considerable extrapolation is necessary. Recent results obtained by observation of satellite orbits extend to altitudes sufficiently high to permit direct inference of physical parameters of the atmosphere at the critical level. It is the purpose of this communication to report such results from the data of King-Hele and Walker4; the salient feature which emerges is a diurnal variation in the physical parameters associated with the escape layer.